As a switching element for an inverter, a transistor, a thyristor and a gate turn off thyristor are used. Since a transistor is superior to the others in characteristics of on and off switching operation, speed, etc. the transistor is used widely according to increasing a rated current thereof. As a transistor has not so large an overcurrent capacity, it is necessary for an overcurrent protection to be provided. For this purpose, as shown in Japan Patent Laid-Open No. 59-103567, a transistor is turned off without regard for the on or off signal being provided for the inverter operation to the transistor when an overcurrent flowing in the transistor is detected. By the system, the transistor is protected from destruction due to the overcurrent. However, there is another problem in the system. the switching speed of the transistor is very rapid, so that the transistor may receive very high voltage due to an inductance being in the circuit including the transistor. The voltage V is defined as follows: EQU V=K.multidot.L.multidot.(dI/dt)
where
K: a constant due to a construction of the circuit,
L: a inductance beng in the circuit, and
dI/dt: a differentiation in time of the current flowing the circuit.
As known from the equation, the current in the circuit is large, especially when a plurality of transistor are directly connected in parallel in order to increase a rated current of the switching element, and the voltage becomes very large. This means that the transistor may be protected from an overcurrent but it may be destroyed by the overvoltage due to an operation for overcurrent protector.
To prevent the destruction of the transistor due to the overvoltage, it is known to employ two methods. One is that the inductance in the circuit is made as small as possible. The other is that a capacitance of a capactor in a snubber circuit to suppress the surge voltage as shown by above equation is made as large as possible. However, both methods are not practical, because the inverter may be very expensive and very large in size.